Plants possess a highly sensitive molecular light swith phytochrome. Red wavelength light activates the Pr form of the photoswitch molecule, which is converted to the Pfr form by the red light absorbed. The Pfr- phytochrome then triggers two types of cellular signal transduction in plants, one at the transcriptional level of regulation for several genes that are not transcribed in the dark and another involving organelle movement and morphological/circadian rhythms such as germination, flowering, etc, some of which may be preceded by transcriptional regulation. The Pfr form (switch-on conformation) can be reverted to the Pr form (switch-off conformation) by far-red wavelength light, as shown below. The molecular mechanism of the phytochrome-mediated photo-signal responses in plants has not been fully elucidated. The long term objective of the proposed study is to understand the mechanisms of cellular light signal transduction at the structural-mechanistic level. Our specific aim of the proposed research is to understand the light- activation of phytochrome and its functional role at the molecular level and use this system as a model to explore the molecular mechanisms of signal switching in general. For these aims, we will use site-directed mutagenesis to probe the interactions between the chromophore and the light-responsive domains of the phytochrome molecule. We will investigate the light-modulated phosphorylation/dephosphorylation of phytochrome as a possible initial event in the light signal transduction cascade. Finally, we will develop an in vitro assay of phytochrome activity based on the possible involvement of G-proteins in the phytochrome-mediated signal transduction in plants, in analogy to the visual excitation process and hormone/receptor-regulated gene expression in animals.